Built on a rhythmic foundation of walking bass and a steady cymbal, Bop features extended soloing over a series of chord changes. In the early 1940s, Charlie Parker and Dizzy Gillespie revolutionized jazz with their stripped-down aesthetic, fast tempos, and harmonic innovations. By the '50s, post-war atomic fear led the music into the anxious and tough sounds of Hard Bop. Outfits such as Art Blakey & the Jazz Messengers, who incorporated elements of Gospel and blues into slightly slower tempos. Post Bop groups, such as the Bill Evans Trio and mid-'60s Miles Davis Quintet, reshaped the basic elements of Bebop and Hard Bop.